1. Field of the Invention
The present invention relates to an injection molding machine of locally pressing type.
2. Description of the Prior Art
Heretofore, there have been proposed injection molding methods that resin in molds is locally pressed to apply a variety of processing operations on molded products with a view to eliminate the need of punching molded sheets or finishing the cut surfaces.
FIG. 1 shows successive steps of one conventional injection molding method of locally pressing type. In the drawing, (a) shows a state under molding, (b) a state under vibration applied for punching, and (c) a state after punching with vibration, respectively.
In FIG. 1, denoted by reference numeral 1 is a fixed mold attached to a fixed platen (not shown), 2 is a movable mold attached to a movable platen (not shown), 3 is a cavity defined between the fixed mold 1 and the movable mold 2, 4 is a sprue through which resin is delivered after being pushed out of an injection nozzle (not shown), 5 is a gate formed between the sprue 4 and the cavity 3, 6 is a fixed-side punch provided on the side of the fixed mold 1, and 7 is a movable-side punch provided on the side of the movable mold 2.
Both the fixed-side punch 6 and the movable-side punch 7 are vibrated by a vibrating device (not shown) disposed in the movable mold 2 during a cooling step after a dwell pressure application step in the course of an injection molding cycle. Since the fixed-side punch 6 and the movable-side punch 7 are arranged beforehand at a position to be punched or a part of the gate 5 where the molded product is to be processed, the vibration of the movable-side punch 7 produces heat due to shear stress in the portion to be processed (i.e., the portion to be punched or the gate portion 5) sandwiched between the movable-side punch 7 and the fixed-side punch 6, so that the resin is softened with the heat, as shown in (a).
By pushing the movable-side punch 7 in one direction as shown in (b), for example, while applying vibration under the above condition, the softened resin is cut out as shown in (c). In this case, the portion cut by movement of the movable-side punch 7 is finished with vibration and cooled to provide the processed surface having a luster and high accuracy.
However, the above-stated conventional injection molding method of locally pressing type is problematic in the complicated mold structure and the increased cost because the vibrating device is disposed in the movable mold 2 and operated to vibrate both the fixed-side punch 6 and the movable-side punch 7.
A further problem is raised in that the resin is cut out by vibrating the fixed-side punch 6 and the movable-side punch 7 during the cooling step after the dwell pressure application step so that the injection molding cycle is elongated.